The present invention relates to matching layers. In particular, conductive acoustic matching layers that are used in sonic or ultrasonic transducer architectures or fabrication.
Acoustic matching layers provide acoustic impedance in between the typically high acoustic impedance of a transducer, typically incorporating a piezoelectric ceramic, and a subsequent medium with different acoustic impedance for the effective transmission of acoustic waves. In medical ultrasound applications, the patient represents relatively low acoustic impedance and the application of 1 or more matching layers provides better matching of the acoustic impedance for acoustic wave transmission between the transducer and the patient. Typically, matching layers are manufactured from non-conductive materials, such as polymers (e.g., epoxies or urethanes). The matching layer may include additional filler materials, such as metal or ceramic filler material, to increase the density and so generate the desired acoustic impedance to create or optimize the transmission of sound energy.
To make a non-conductive acoustic matching layer conductive, conductive filler is dispersed within the matching layer. The filler may be shaped and sized, such as providing needle like, or whisker type shapes. The filler is positioned randomly within the matching layer. High concentrations of electrically conductive fillers are provided for particle-to-particle contact throughout the bulk of the matching layer. The particle contact allows electrical conduction though the material. However, the high concentrations of filler result in higher acoustic impedance, rendering the matching layer less useful for matching the impedance of the transducer ceramic to the relatively low impedance of a patient, especially in multi-matching layer designs where the outermost matching layer is typically a relatively low impedance, such as less than 3 Mkayl.
As an alternative to a conductive filler, a conductive material, such as a solid graphite, magnesium, or conductive polymer chain may be used for the matching layer. However, solid materials such as graphite tend to have relatively higher or very specific acoustic impedances, limiting the usefulness of such matching layers. Graphite or other solid materials are machined, making the material less convenient than a castable polymer material for manufacturing curved parts. Conductive polymer molecules are typically modified (i.e., loaded) and rarely inherently conductive. The physical properties are limited for suitability in transducer applications and adequate conductivity.
For one-dimensional transducers and transducer arrays, conductivity between the upper and lower transmission surfaces of matching layers may be accomplished by a metallic plating or sputtered film on the edges of the matching layers electrically connecting the upper and lower surfaces. For one-dimensional transducer arrays, the sides of the matching layers are easily accessed for sputtering or plating. However, for multi-dimensional arrays, such as 1.5 or 2dimensional arrays, circumferential plating or sputtering is difficult to use due to the limited access to the sides of the matching layers of each element.
Phased 1.25, 1.5, 1.75 and 2 dimensional ultrasound arrays include a plurality of array elements in the elevation and azimuth dimensions. For a large steering angle, such as used with two-dimensional phased arrays, the elements desirably have acceptance angle and little or low electric and mechanical crosstalk both in elevation and the azimuth dimensions. Dicing is used to mechanically separate individual transducer elements to minimize the mechanical coupling or crosstalk. For example, one dimensional arrays typically have one or more acoustic matching layers positioned between the PZT ceramic and the lens or patient. The PZT and matching layers are diced in 1 axis separating individual array elements to reduce mechanical crosstalk through the matching layers. Electrical connections are provided to PZT along the edges of each array element.
For phased two-dimensional arrays, dicing is required in both the azimuth and elevation dimensions to reduce crosstalk. Either one or no electrically conductive, high impedance matching layers are stacked on top of the PZT ceramic and separated into individual elements. A common ground foil or signal-flex is laminated above the PZT and any electrically conductive matching layer, typically perpendicular to desired sound wave transmission to provide a second electrical connection to the PZT. The connecting conductive layer cannot be physically separated, as are the individual elements, in both dimensions if it is to provide external connection elements in the array. Electrically non-conductive matching layers are then laminated above the ground foil or signal-flex. The non-conductive matching layers provide a lower acoustic impedance. The non-conductive matching layers may additionally be diced in the azimuth and elevation dimensions. However, by using no matching layers or only one electrically conductive matching layer, reduced axial resolution and lower bandwidth result. Where additional non-conducting matching layers are provided but not diced, crosstalk increases and the acceptance angle is reduced. If the additional non-conductive matching layers are diced, an additional dicing process step results, and alignment issues may result. Crosstalk cannot be optimally reduced, since the acoustic matching layers cannot be entirely diced without risking cutting signal traces or the ground foil.